The question specifies a number of important facts, and many of the answers here have challenged those facts' potential accuracy. I'll try, conversely, to accept the limitations proposed and see if that's helpful.
The strictures, as I understand them, are as follows:
- Magicians can create gold, silver, gems, etc.
- These products are indistinguishable from their mundane counterparts.
- These goods are held to be valuable as currency.
Given all this, what might be done to prevent economic disaster?
First off, a corollary of the extraordinary power of magic as well as stricture #2 above: can magical means detect a difference? For example, can magical means be used to discern the history of a given object, its location of origin, etc.? If so, those who can afford to do so and have a vested interest in the stability of the economy (banks, governments, etc.) may well have wizards whose job it is to detect counterfeits.
But let's suppose that #2 is true in a radical sense: the created gold is in that case not a forgery. It's perfectly real. In that case, it seems to me that the issue devolves into one of interests.
On the one hand, there are those who have a strong vested interest in the long-term stability of the economy, such as banks and governments and so forth. You might say that everyone has such an interest, but that isn't necessarily the case. If I find a huge nugget of gold in my backyard, my inclination is to turn it into money (sell it to a bank or gold dealer, etc.); the fact that this may in some sense devalue the world's gold reserves isn't something I'm going to worry about -- I leave that to the world's financial industry, and if they have some suffering because of it, well, sounds good to me. Not my problem.
Now in early modern Europe, particularly in the sixteenth century, alchemy was big business. That isn't just turning base metals into gold, to be sure, but let's focus on that for the sake of the question. One way of dealing with alchemists was to punish them: the reasoning here is not that the gold they create is necessarily not real, but that the Crown has the monopoly on creating currency, and as a result, gold-making is arguably a crime of lèse-majesté. Another possibility -- equally attested -- reverses this reasoning: if this alchemist seems likely to be able to pull it off, the Crown wants him working for the Royal Mint. So in the end, it's a matter of interest.
1. Who has a strong and indeed vested interest in the totality of the world economy?
Until quite modern times, nobody. If I have more gold than you do, whether "I" and "you" are individuals, guilds, or states, then I'm richer than you are, period, no matter where I got the gold. To be sure, the history of Spanish conquest and the import of vast amounts of gold and silver to Europe from the New World did cause tremendous damage in the long run, but in the short run it certainly made Spain very wealthy. And although we might now argue that this incredible surge of precious metals was a bad idea, the Spanish didn't see it that way at the time. So by this reasoning, if nobody is particularly looking out for the world economy, the only thing stopping magicians from creating gold is local interest.
2. What do local interests think about creating gold and such?
Depends how easy it is, and how firmly the Crown or whoever holds on to its currency monopoly. If there are zillions of magicians running around doing this all the time, then I do think the value of these goods would have collapsed long ago, but the question as posed presumes otherwise; this suggests to me that producing gold magically is rather rare. In that case, magicians who can do this are themselves resources, like gold mines. So by this reasoning, the laws and such governing magicians creating gold will tend to approximate those governing gold mines, the differences being primarily that (a) magicians can move somewhere else, and (b) magicians can be killed. If I were the Crown, I think I'd want to link a to b: the magician works for me in the Mint, and does what I tell him to, and if he ever tries to move away I'll have him killed.
3. What wider implications does this have?
3a. If I'm a magician who can make gold, I either do or do not want to work for a state. Could be a very cushy job: the Crown doesn't want to have to kill the goose that lays the golden eggs, and it's not terribly difficult for the Crown to ensure that I lead an awfully pampered life if it wants something from me. So if I have that power, I may well seek patronage of this kind -- many alchemists in European history did precisely this. If for some reason I really badly don't want to work in this way, perhaps because I like traveling around, then I had better not advertise that I can create gold this way.
3b. Suppose there are three states, which for sake of simplicity we'll call France, Spain, and England. Now Spain has a whole lot of powerful magicians working for them, and they're cranking out gold and silver like nobody's business (cf. Spanish New World colonial exploitation). England and France would certainly like to have that same power, but they don't, so what do they do? France might well try to cozy up to Spain, do them favors and whatnot in order to get Spain to loan them a magician or give them gold cheap. England, on the other hand, might decide instead to kidnap Spanish magicians -- or even assassinate them (cf. licensed privateering in the Caribbean).
The central point is that magicians as described in the question are effectively moving gold mines. The powers that be will want to control these mines, and if they cannot do so, they will want to ensure that nobody else controls them instead. I do not see a global concern for "the economy" being a realistic issue in the absence of a global financial industry. If such an industry does exist, then it will act in much the same way as any ordinary state: it will want to control the moving gold mines, and ensure that nobody else controls them. It is certainly possible that such an industry will make the claim that it must do this to protect everyone in the world from ruin, but somehow I suspect that the people who run that industry will get remarkably wealthy as they achieve greater and greater control over the magicians.